CNN is showing some kind of program about terrorism, right now, and one of the people they've interviewed said something to suggest that it was not all that difficult to construct a nuclear bomb. I think this person was wrong.

Nuclear bombs are very difficult to design. The technical problems faced by someone trying to construct such a device are staggering and are too complicated for me to cover here. But I have read about them in a book called "Critical Assembly : A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945". This book does not provide the kinds of techical details about the solutions to problems faced by those on "Project Y" to tell the reader how to construct a nuclear device; But, it does give enough detail about those problems to impress the reader with just how difficult these devices were to design. After reading this book, I am convinced that it is very unlikely that a terrorist organization cannot successfully design a nuclear bomb without a great deal of money, the protection of a government, and a high risk of being detected by Western nations. But if you're worried that I'm wrong, read the book, yourself. I think you'll find it convincing.

The first time you invent something from scratch, it can be incredibly difficult. But once the cat is out of the bag (the knowledge that it can be done), there are only a couple of really hard problems to solve and a small team of engineers and physicists could design and build such a device. Many of the original problems were the result of lack of computing power. Some of the remaining problems include modeling the implosion characteristics* and producing the correct high explosive ‘wrapper’ and obtaining and working with the nuclear materials themselves. There are several high tech and specialized components to be procured also. It’s also simpler to use an obsolete ‘gun assembly’ design.

The difficulty lies not so much in the actual fabrication, but in all the preparation required, setting up the right shop, obtaining the material, testing some sub assemblies and keeping the team together and secret from the eyes of the international community.

If I could steal or buy the nuclear material, was willing to forego testing, and did not have to operate in secret, it would not take that much effort or money (a few tens of millions). Of course my design would have an somewhat unpredictable yield.

So in essence you are correct that the limiting factors are secrecy and detection during your design and development cycle, rather then design hurdles. The design hurdles are just not that high.

* the computing horsepower now exists on any basic PC, but the software would have to be stolen or written from scratch.

I think you are right about the gun device being simpler, but I also think you are underestimating the number of problems that must be solved, even with the gun device. As examples:

the initiator must be designed.

Purification of the initiator materials is also complicated, and getting the fissionable material from an external source doesn't solve the problem of aquiring initiator material

the timing of setting off the gun and the initiator must be determined.

critical mass is dependent on the density and geometry of the nuclear material. I remember that plutonium has different densities depended on how it is purified. Is the same true for uranium? What will be the exact shape of the combined mass of nuclear material be when the gun has finished bringing them together? Will the nuclear material be compressed at all by the gun? That would also affect critical mass.

Uranium is a difficult metal to work with? How is it to be shaped properly?

In addition to all these points, I think gun devices are harder to make small.

It easy to say that something isn't very hard. But for this kind of device, I think it is still very difficult to solve all the problems required for putting together a good design. Certainly there are more than a couple of problems to solve.

My reference to specialized components was an oblique way of bringing up the initiator, one of the hardest things to build, and you are exactly right about initiator timing being critical for pre and post detonation reasons. Also, you are correct in that the gun design cannot be made small on the same scale as an implosion design because the gun does not really compress the uranium, it just assembles a 'normal density' critical mass. It's an inefficient design from a 'use of nuclear materials' point of view. I believe (but I could be wrong) that both Uranium and Plutonium are cast in 'near net' shape and then machined. None of these is an insurmountable problem, just challenging in multiple different fields. I agree with you.

Here's a cool link, if you don't already have it...

I have taken to studying this type of nuclear physics area a lot over the years, both due to my (life long) geeky scientific curiosity, and my desire to be able to have educated guesses about proliferation like you are discussing here.

I just friended AdrienG back and look forward to reading you. Any friend of Malabar... It is my usual practice to friend anybody back unless casual inspection of their journal scares me, heh!